30 ANATOMY AND PHYSIOLOGY 



while in other cases, as in Figs. 67 and 68 in the same 

 Plate, they are arranged in verticillate order on all parts of 

 the spiculum. In each of these varieties the bases of 

 the spicula are usually profusely furnished with spines 

 so as to ensure a strong and somewhat rigid mode of 

 attachment. 



There is undoubtedly a special purpose in every variation 

 of the spination of these spicula, and in their presence 

 generally. The short strong form and acute distal termi- 

 nation admirably adapts them to encounter the larger 

 description of intruding annelids, the most dangerous 

 internal enemies of the Spongiadse; while the spination 

 of their shafts presents a series of minute weapons that 

 would prove equally formidable to those intruders that 

 were too minute to be affected by the larger weapons 

 of defence. 



The acuate entirely spined defensive spicula are of very 

 common occurrence in sponges, and are by no means 

 confined to particular tribes or genera. As a general 

 rule, when the external defences are very full and suffi- 

 cient, we should not expect to find the internal defences 

 abundant, and, on the contrary, when there appears to be 

 a paucity of external defences, the internal ones are fre- 

 quently exceedingly numerous. Thus, in the genus 

 hictyocylindrus, Bowerbank, where in almost every species 

 the surface of all parts of the sponge is bristling with the 

 acute terminations of the radiating external defensive 

 spicula, although in most of the species we find acuate 

 entirely spined internal defensive ones, yet in many of 

 the species they are so rare as to be by no means readily 

 detected. 



When the skeleton is formed of keratose fibres, we find 

 them dispersed on their surface without any approach to 

 order, and projected at every imaginable angle. If the 

 skeleton be formed of any of the varieties of spiculous reticu- 

 lations, they are based in a similar manner on the prin- 

 cipal lines of the reticulated structure, and sometimes but 

 not very frequently, they occur in groups. 



I will not extend this portion of my subject to an 



